Revealing mistake: When Nuclear Man melts the rifle, note that the SWAT member is holding the barrel and he slides his hand away slowly to make the barrel bend downwards. He was obviously holding it up. If he had burned himself he would've moved faster.
Revealing mistake: When the fire truck explodes, note a tube behind it going straight to the truck. Probably an FX device to achieve the smoke.
Continuity mistake: When the firemen are about to grab the hose and it explodes, note that the other hose inside the truck on the right side has changed positions between shots.
Continuity mistake: Nuclear Man walks towards the newspaper where Lucy's picture is, for a brief moment there's a light smoke coming from behind him as if something was burning (it has nothing to do with the smoke Nuclear produces when flying) which is later gone (according to SupermanIV.com a deleted scene involved Nuclear Man waking up, "he sees and picks up the newspaper with Lacy, which then catches on fire in his hand.").
Continuity mistake: Before Nuclear Man see Lacy's picture in the newspaper, he rises from the floor and a wide angle reveals that the table is nowhere around, yet a couple shots later it simply appears half a meter away from him.
Revealing mistake: When dying and balding Clark grabs the green glass, the border of his wig noticeably protrudes all around his face.
Continuity mistake: When Lois visits sick Clark she stands by the door, fully lit, a frame later, when the angle changes, the whole place is in the shadows.
Revealing mistake: When Superman discovers that Nuclear Man is stealing the Statue of Liberty, he turns around and you can see the wrinkles on his temple from the wig he was using.
Continuity mistake: When Superman first meets Mr Warfield, the orange pencil on the table changes positions between shots.
Continuity mistake: When Superman stops the train there are two thick black tubes next to the tunnel behind which disappear when he leaves.
Audio problem: When Lex enters the military bunker, officer Goram's voice is obviously dubbed and sometimes the audio doesn't match his lips.
Continuity mistake: The white box that Lex attaches to the missile, containing Nuclear Man's 'ingredients', appears and disappears between shots.
Continuity mistake: When Nuclear Man is born, both hands are clenched. He opens and extends the right one first, then the left. A frame later the right hand is slightly clenched again.
Continuity mistake: The Daily Planet Building lobby is a completely different one between the scenes where Nuclear Man destroys Metropolis and the one where Warfield is fired.
Revealing mistake: After the power plant 'swallows' Nuclear Man, the needle meters have the word AUTODIESEL UXBRIDGE written inside. Uxbridge is the British city where the scene was shot, therefore not the fictional Metropolis.
Continuity mistake: When Superman throws Nuclear Man down the power plant, Nuclear's black wristbands disappear and reappear between shots.
Revealing mistake: Obvious doll replaces Nuclear Man when he falls into the chimney.
Visible crew/equipment: Obvious stunt double for Christopher Reeve when lifting the lady to the China Wall; note his thick sideburns, big forehead and skinny build.
Continuity mistake: When Superman catches the falling girl by the Big Wall, his hair is pointed upwards, obviously due to a big amount of hair fix to simulate a moving hair. A frame, later, from another angle, the hairstyle is different, no sign of the hair fix.
Revealing mistake: When Superman meets Nuclear Man at the terrace, the blue-screen surrounding the set is reflected on the windows.
Answer: There appears to be multiple reasons. They had creative differences, ultimately resulting in a poorly received movie. Kidder said Reeves, who co-wrote the story, had an inflated ego and clashed with Furie.
raywest ★
Moreover, Mark Pillow, who played Nuclear Man, claimed that working with Reeve was quite intense.